In the hydrostatic working operation of road finishing machines or feeders, the internal combustion engine is traditionally operated at a fixed speed level, independent of how much hydraulic or electric energy is consumed by working components. The reason for this is the ambition confirmed by experience to keep all parameters constant and not to provide any flexibility for the operation of the internal combustion engine as former closed-loop control technology and the reaction of the driver did not permit to adapt to changing laying situations quickly enough. This brings about that engine operation requires high specific fuel consumption mainly with small loads of the internal combustion engine at a fixed high speed level, and that this high speed level results in a considerable amount of environmental and noise pollution. With such load conditions, moreover unnecessary drag losses result, for example due to fans and hydraulic pumps which also increase fuel consumption. It is already known to adjust the speed of the internal combustion engine dynamically, for example by the driver, when the road finishing machine or feeder is driven for transport. This is done manually and independent of the load condition of the internal combustion engine.
WO 2010/006759 A1 recommends to load-dependently change the speed of the internal combustion engine during the operation of a road finishing machine such that the speed or the frequency of at least one drive is at least approximately kept constant. For this, two different strategies are employed. In the one case, energy supplied to certain drives of the road finishing machine is kept constant in case of a reduced energy demand by diverting excessive energy to other drives which do not require a constant speed. Conversely, if the power of at least one certain drive of which the speed must be kept constant is increased, energy is taken off and diverted from other drives which do not require a constant speed. In the second case, if the energy diversion is no longer sufficient, in case of an increase of power delivered to a drive requiring a constant speed, its energy supply is increased by increasing the speed of the internal combustion engine. Conversely, if the power demand of a drive requiring a constant speed drops, the speed of the internal combustion engine is reduced. In each case, the hydraulic volume flow rate is changed, while the hydraulic pressure, however, remains constant. Thus, the volume flow rates are changed for the drives requiring a constant speed, the course of the closed-loop control being such that first a speed drop or a speed increase is permitted and waited for within a predetermined speed frame before the control goes against it.
Furthermore, from DE 10 2008 058 174 A1, a closed-loop speed control system for the internal combustion engine of a compacting vehicle, e.g. a compaction roller, is known wherein pressure sensors are employed for determining the consumed hydraulic power of hydraulic motors supplied by variable displacement pumps. Closed-loop speed control is accomplished by compensating the actually required hydraulic power and the power output by the diesel engine. Utilizing the signals of the pressure sensors, by compensation, a signed difference between the required hydraulic power and the power output of the internal combustion engine is detected, and the speed of the internal combustion engine is controlled in response thereto. The displacement of the respective variable displacement pump is manually adjusted to a certain value and can be changed by the driver if he detects it to be inappropriate.
Further prior art can be found in US 2007/150166 A and JP 2002 039111 A.